Imaging output such as color photographs, ink jet images or thermal dye transfer images are typically designed to be flat and planner. A flat, planner image is desired, as the viewing and handling of the image is optimized and preferred by the consumer. There is, however, a need for an image that can be formed into a non planner shape without the need for mechanical constraint of the image.
Printing media are improved if they resist curl and remain flat. This is particularly important with media utilized with the formation with color images. Such media include those utilized for ink jet, thermal dye sublimation imaging, thermal wax dye transfers and electrophotography. The formation of a flat base is particularly important for color photographic paper. In color papers there is a need for providing color papers with improved curl. Present color papers will curl during development and storage. Such curl is thought to be caused by the different properties of the layers of the color paper as it is subjected to the developing and drying processes. Humidity changes during storage of color photographs lead to curling. There are particular problems with color papers when they are subjected to extended high humidity storage such as at greater than 50% relative humidity. Extremely low humidity of less than 20% relative humidity also will cause photographic papers to curl.
Curl values are the measurement amount of deflection in a parabolically deformed sample. An 8.5 cm diameter round sample of the composite is stored at the test humidity for 21 days. The amount of time required depends on the vapor barrier properties of the laminates applied to the moisture sensitive paper base, and it should be adjusted as necessary by determining the time to equilibrate the weight of the sample in the test humidity. The curl readings are expressed in ANSI curl units, specifically 100 divided by the radius of curvature in centimeters.
The radius of curvature is determined by visually comparing the curled shape, sighting along the axis of curl, with standard curves in the background. The curl may be positive or negative, and for photographic products, the usual convention is that the positive direction is curling towards the photosensitive layer. It would be desirable if the humidity curl for photographic print paper could be reduced.
Color print papers are usually made of three structural layers under the emulsion; raw paper base and a top and bottom coating of extrusion cast polyethylene. Since the strength properties of polyethylene in this form are poor in comparison to the raw base, the curl properties or the stiffness of a print in bending are determined almost exclusively by the raw base. Because of the alignment of the paper fibers during manufacturing, the raw base is highly oriented with respect to strength properties in the plane of the sheet, for example, the ratio of the elastic modulus in the strongest to weakest direction is often 2:1, with the strongest direction in the MD (machine direction). This is why the shape of a typical polyethylene sample with curl, caused by resisting emulsion shrinkage on one surface at low humidity, is cylindrical with the axis of curl along the MD, so the sample curls in its weakest direction, or directly in the CD (cross direction).
It has been proposed in U.S. Pat No. 5,866,282 (Bourdelais et al) to utilize a composite support material with laminated biaxially oriented polyolefin sheets as a photographic imaging material. In U.S. Pat No. 5,866,282, biaxially oriented polyolefin sheets are extrusion laminated to the top and bottom sides of a base material to create a support for silver halide imaging layers. The biaxially oriented sheets described in U.S. Pat. No. 5,866,282 have a microvoided layer in combination with coextruded layers that contain white pigments. The composite imaging support structure described in U.S. Pat. No. 5,866,282 has been found to be more durable, and more tear resistant, sharper and provide brighter reflective images than prior art photographic paper imaging supports that use cast melt extruded polyethylene layers coated on cellulose paper. During the lamination process, in which the biaxially oriented sheets are adhered to the base materials, the biaxially oriented sheets can be heated or cooled prior to lamination. Heating or cooling prior to lamination causes dimensional changes of the biaxially oriented sheet that return to the original dimension after the biaxially oriented sheet have returned to ambient temperature.
It has been proposed in U.S. Pat. No. 5,244,861 to utilize biaxially oriented polypropylene laminated to a base paper for use as a reflective imaging receiver for thermal dye transfer imaging. While the invention does provide an excellent material for the thermal dye transfer imaging process, this invention cannot be used for imaging systems that are gelatin based, such as silver halide and ink jet, because of the sensitivity of the gel imaging systems to humidity. The humidity sensitivity of the gelatin imaging layer creates unwanted imaging element curl. One factor contributing to the imaging element curl is the ratio of base paper stiffness in the machine direction to the cross direction. Traditional photographic base papers have a machine direction to cross direction stiffness ratio, as measured by Young's modulus ratio, of approximately 2.0. For a composite photographic material with biaxially oriented polyolefin sheets laminated to a base paper, it would be desirable if the machine direction to cross direction stiffness ratio for the paper were approximately 1.6 to reduce imaging element curl.